The present invention relates to a laser arrangement and to a method for enhancing the life span of optical elements in a laser arrangement.
The invention is particularly suited for laser arrangements in the form of optical frequency converters.
Optical frequency converters are employed in order to convert the frequency of a laser radiation in spectral ranges in which generating a laser radiation directly is not possible. Laser light in the deep UV range (wavelength below about 300 nm, particularly below about 290 nm, particularly below about 280 nm, particularly below about 270 nm, and particularly up to approximately 120 nm, particularly up to approximately 110 nm, particularly up to approximately 100 nm) is employed in the semiconductor industry for wafer inspection systems, among others.
Various laser arrangements are known from prior art which can generate laser light in the deep UV range by means of frequency conversion with the help of non-linear crystals. The resonant frequency conversion in a passive optical resonator has proven to be the most efficient method for frequency conversion of continuous (CW-) laser radiation (Ashkin et al. “Resonant Optical Second Harmonic Generation and Mixing”, Journal of Quantum Electronics, QE-2, 1966, page 109). Therein, the laser beam to be converted is coupled into an optical resonator which contains a non-linear crystal suited for frequency conversion as well as resonator minors. The resonator is set to resonance with the radiated laser frequency by a corresponding setting of the optical resonator length. The thereby attained resonance magnification of the laser radiation circulating in the resonator allows for a high conversion efficiency into the laser radiation with the desired converted frequency. Magnification factors of up to approximately 100 and conversion efficiencies of 20% up to more than 80% are attained here, depending on the wavelength and capacity of the radiated laser light.
When generating laser light in the deep UV range, the optical elements involved, such as, for instance, the non-linear crystal or the resonator minors, are exposed to high strains due to the aggressiveness of the UV light, which leads to a degradation on the surfaces as well as inside of the material of the optical elements. Consequently, the life span of UV lasers mostly does not meet the high demands of industrial applications, which, particularly in the semiconductor industry, are generally at least 20 000 hours in continuous operation.
The degradation of the optical elements progresses the faster the higher the intensity of the UV light that acts on them. The strain on the non-linear crystal is generally the highest, because in the interest of a high conversion efficiency the beam cross section is smallest there. As the conversion efficiency in the non-linear crystal increases with the power density of the radiated laser light, in the interest of a long life span the beam cross section cannot be chosen arbitrarily large. For this would lead to a reduced power density and, hence, to an unattractive conversion efficiency. Consequently, choosing the beam cross section is always a compromise between a long life span on the one hand and a high conversion efficiency on the other hand.
A method for extending the life span without the undesirable reduction of the conversion efficiency is specified in U.S. Pat. No. 5,825,562. Therein, an optical element exposed to the UV radiation is permanently moved with the help of a mechanical device, whereas the UV beam keeps its position, so that on average the strain is distributed onto a larger portion of the optics material and the degradation is thereby slowed down.
In U.S. Pat. No. 6,859,335 the optical element is not moved permanently, but instead step by step in quick succession, wherein the step sequence shall be shorter than the typical degradation time of the material (namely in the range of milliseconds to minutes) and the same material points are used over and over again.
In both these methods known from prior art, optical elements of the laser are moved during the running operation.
Applying these methods for extending the life span to CW-laser devices with frequency conversion in the deep UV range proves to be problematic in various regards.
For instance, the motor-driven, continuous movement of an optical element within a passive resonator can lead to vibrations and thus to periodic intensity fluctuations. As the demands on the intensity noise of CW-laser devices generally are very high, this can lead to unacceptable high noise values or to a disproportionally high expenditure to prevent these vibrations, respectively.
Even the gradual movement of optical elements in quick succession can cause unacceptable disturbances of the temporal intensity process, when the succession of the disturbances takes place in shorter time spans than the process time needed during the application. Thus it is, for instance, unacceptable when within the process time needed for inspecting a wafer short intensity slumps of the UV radiation occur. The gradual movement of optical elements within a passive resonator, however, can easily provoke such intensity slumps.
Furthermore, the life span of a CW-laser device having a passive resonator for the frequency conversion is not only determined by a single optical element. In a passive resonator according to prior art, as it is shown in FIG. 4, a converted laser beam 2 generated from a first laser beam 1 exits a non-linear crystal 3 and then runs through an output coupling mirror 4, which has a high transmission for the wavelength of the converted laser beam whereas it is highly reflecting for the wavelength of the first unconverted laser beam 1.
When the non-linear crystal 3 is moved to a new position 3′ with the help of suitable mechanics, the life span of the non-linear crystal is indeed enhanced, not, however, the life span of the output coupling mirror 4. Said output coupling mirror 4 is under a similar strain as the non-linear crystal 3, because the beam diameter and, consequently, the power density of the UV radiation are not substantially different from the power density at the location of the crystal 3. The life span of the entire system can therefore not be enhanced substantially by exclusively shifting the non-linear crystal, as the life span of the output coupling mirror generally is only approximately by a factor of 2 longer than the life span of a point in the crystal 3 that is subjected to UV radiation. However, the output coupling mirror 4 must not be moved along with the shifting mechanics because otherwise the passive resonator would come out of adjustment and the conversion efficiency of the arrangement would thereby immediately break down.